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Nitrates are a by-product of the natural processes that occur in both nature and in the aquarium. They are caused by such things as too many fish, dirty filters and decaying plant matter. When the conditions that produce nitrates are kept to a minimum, nitrate production is small. Good management habits including regular cleaning and water changes can help to prevent problems and to reduce nitrates in aquariums to acceptable levels. Special filtration systems can also be helpful.
Normal freshwater and saltwater aquarium nitrate levels should be below about 10 parts per million (ppm). Levels higher than that can stress young or sensitive fish and will promote the growth of algae. It is possible for fish to survive with higher levels, but this is not recommended. New fish that are introduced to an aquarium high in nitrates may become ill or may be unable to survive.
One of the most important things you can do to reduce nitrates in aquariums is simply to keep the aquarium clean. Hobbyists who overfeed or who try to place too many species into an aquarium are at risk of losing their fish. The decaying food and vegetation produces nitrates as part of the rotting process, and these get into the water where they can cause problems for the fish.
A significant way to reduce nitrates in aquariums is through regular water changes. Changing out small amounts of the water each week may impact the nitrate level, but most likely won’t help enough to drop the levels by much. For best results, about half of the water in the aquarium should be replaced at once. The new water should be tested for nitrates before being added, and the nitrate levels in the tank should be checked afterward.
You can also purchase special filters that are able to remove nitrates from the water. These filters rely on live bacteria to reduce nitrates in aquariums and work well once they are established. The removal process is slow to start but works well once it is up and running, though the filters can be a bit tricky to use.
Live plants are an excellent aid to help you to reduce nitrates in aquariums. Since decaying plant matter can cause nitrates, it is important that the plants in your tank be vigorous, healthy and growing. In a freshwater system plants can be planted in the gravel on the bottom of the tank. In a saltwater aquarium the best choice is usually to add live rocks, which are rocks with plants growing on them.